The present invention is directed towards an ultrasonic detection circuit which is capable of determining the condition of a fluid within a container without entering the container. More particularly, the present invention relates to an ultrasonic circuit which applies an ultrasonic pulse to the outer wall of the fluid container and determines the condition of the fluid within the container as a function of the decay rate of the sonic pulse as it is reflected between the inner and outer wall boundaries of the container wall. While this circuit is capable of determining many types of fluid conditions (such as the density of the fluid), it is preferably used to determine the presence or absence of liquid within the container.
It is frequently desirable to determine whether a pipe or container is filled with a liquid or to determine the height of the liquid within the container. This is commonly done by gages or other equipment which must penetrate the container wall and reach into the interior of the pipe or the container. These gages are then used to produce an indication of liquid level or the presence of liquid in the container, and can be used to operate liquid flow or liquid level control. The need to gain access to the container interior has the obvious drawbacks of requiring special container designs and, in some cases, special seals to bring out gage components, electrical wires, and the like. Moreover, these parts become subject to failure when exposed to corrosive fluids and hostile environments within a liquid container.
In U.S. patent application Ser. No. 822,199, of which the present invention is a continuation-in-part, a novel ultrasonic measuring circuit which can determine the foregoing conditions without penetrating the container wall is disclosed. In accordance with the foregoing application, the longitudinal ultrasonic energy pulse is applied to the outer boundary of the container wall and undergoes multiple internal reflections within the container wall between the inner and outer boundaries thereof. The amplitude of these reflections decays at a rate dependent upon the sonic impedance of the medium within the container adjacent the inner surface of the container wall in the area of the wall in which the ultrasonic pulse is introduced. If the container is empty at the point where the measurement is made, the rate of decay of the interface reflections is lower than if the pipe is filled with liquid at that point. Thus, by measuring the rate of decay of the amplitude of the multiple reflections, it is possible to determine whether or not there is fluid within the container at the point where the measurement is made. Since the sonic impedance of various liquids differ, a measurement of the rate of decay also may permits the identification of the liquid in the container.
In the foregoing application, the rate of attenuation of the multiple reflections is measured by measuring the exponential decay of the peak value of the reflections. While this approach is suitable for most applications, it has been found unsuitable in thin wall containers and where adequate perpendicularity between the transducer axis and a tangent to the container surface cannot be maintained. In these cases, the peak values of the reflections made depart from a "perfect" exponential with a result that improper determinations may be made by the detector circuitry.